1. Technical Field
The present invention relates to a voltage control type temperature compensation piezoelectric oscillator used in a communication device and the like. Especially, the voltage control type temperature compensation piezoelectric oscillator is preferably used for improving a shift of a temperature compensation voltage caused by variation of a control voltage.
2. Related Art
Piezoelectric oscillators are used in various areas such as communication devices and electric devices due to their advantageous points such as small size, light weight, and low cost. Especially, a voltage control type temperature compensation piezoelectric oscillator (hereinafter, referred to as VC-TCXO) is widely used, for example, used in a cell-phone. The VC-TCXO compensates a frequency-temperature characteristic of a piezoelectric resonator, and an oscillation frequency thereof can be varied depending on applied voltage.
JP-A-2002-217643 is an example of a related art. The example discloses a temperature compensation voltage generating circuit of a quartz crystal oscillator. FIG. 12 is a block diagram showing a structure of a voltage control type oscillation circuit including: a quartz crystal oscillator composed of a varactor diode 55, a quartz crystal resonator 56, and an oscillation circuit 57; a temperature compensation voltage generating circuit 61; a storage device 62; and an AFC circuit 63.
The temperature compensation voltage generating circuit 61 includes a gain control amplifier (hereinafter, referred to as “AGC”) 68a for controlling an output voltage of a primary voltage generating circuit 59, and an AGC 68b for controlling an output voltage of a third-order voltage generating circuit 60. Gains of the AGCs 68a and 68b are controlled by an output voltage value of the AFC circuit 63.
Commonly, a frequency-voltage (f-V) characteristic of the varactor diode 55 does not exhibit a straight line. Here, a case where an output voltage Vc of the AFC circuit 63 is a center value Vcm is set to be a reference. Under this case, a varying amount Δf of a frequency differs from a varying amount ΔV of the output voltage Vc depending on whether the output voltage Vc is larger than the center value Vcm or smaller.
The circuit is structured such that when the output voltage of the AFC circuit 63 is higher than the center value Vcm, the gains of the AGCs 68a and 68b are decreased, and when the output voltage is lower than the center value Vcm, the gains of the AGCs 68a and 68b are increased.
The gains of the AGCs 68a and 68b are optimized based on the f-V characteristic of the varactor diode 55 so as to make the varying amount of the oscillation frequency as constant as possible within a variable range of the AFC circuit 63. Accordingly, a voltage control type temperature compensation quartz crystal oscillator maintains linearity of an output oscillation frequency for a control voltage, and achieves highly-accurate temperature compensation.
The example discloses that even when the f-V characteristic of the varactor diode exhibits a curve, the temperature compensation voltage generating circuit of the example compensates for the curve and keeps the linearity of the output oscillation frequency for the control voltage as a voltage controlled oscillator (VCO), being able to realize temperature compensation.
A frequency variable part around the piezoelectric resonator of the VC-TCXO is a circuit in which a parallel circuit including a variable capacitance element C1 for compensating the frequency-temperature characteristic of a piezoelectric resonator X and a variable capacitance element C2 for varying an oscillation frequency of the piezoelectric resonator X, and a load capacitance C0 are coupled with the piezoelectric resonator X in series, as shown in FIG. 13.
When the control voltage Vc of the VC-TCXO is set to be at the center value Vcm, a value of the variable capacitance element C2 is uniquely determined. The load capacitance C0 and the like are adjusted under this state so as to adjust the oscillation frequency of the VC-TCXO at a predetermined center frequency. The load capacitance C0 is fixed after the adjustment. Then output voltages of the primary voltage generating circuit and the third or higher-order voltage generating circuit are adjusted to be applied to the variable capacitance element C1 for temperature-compensating the voltages. Thus the temperature compensation is carried out so as to obtain a desired frequency-temperature characteristic.
However, when the external control voltage Vc of the AFC circuit is varied to vary the oscillation frequency of the VC-TCXO as the primary function of the VC-TCXO, an initial value of the variable capacitance element C2 is varied, whereby the frequency-temperature characteristic is shifted from the initial characteristic disadvantageously.
The phenomenon of the shift of the frequency-temperature characteristic is circumstantially described by using numerical values.
In the circuit of FIG. 13, the load capacitance C0 is set to be 30 pF, a value of the control voltage Vc is set to be VcL, and a capacitance value of the variable capacitance element C2 is set to be 5 pF. A capacitance value of the variable capacitance element C1 for temperature compensation is set to vary within a range from 5 pF to 15 pF (C1MIN=5 pF, C1MAX=15 pF) by the output voltages of the primary voltage generating circuit and the third or higher-order voltage generating circuit due to temperature change. Equations of (C1MAX+C2)=CDS and (C1MIN+C2)=CSS are set to be satisfied. In a case where the capacitance value of the variable capacitance element C1 is varied within the range from 5 pF to 15 pF, a varying amount ΔCL of a load capacitance CL of the piezoelectric resonator X is expressed as ΔCL=(CDS×C0)/(CDS+C0)−(CSS×C0)/(CSS+C0). The above numeral values are assigned to this equation, deriving the relation: ΔCL=4.5 pF.
Next, the value of the control voltage Vc is set to be VcS, and the capacitance value of the variable capacitance element C2 is set to be 15 pF. In this case, the varying amount ΔCL of the load capacitance CL of the piezoelectric resonator X is expressed as: ΔCL=3.0 pF.
That is, the capacitance variation, for temperature compensation, of the piezoelectric resonator X differs depending on the value of the control voltage Vc applied to the variable capacitance element C2. As a result, even if the temperature compensation of the VC-TCXO is carried out at the center value Vcm of the control voltage Vc, the frequency-temperature characteristic is shifted (deteriorated) from the initial characteristic when the control voltage Vc is actually varied.